Electronic systems utilize dimmers to modify output power delivered to a load. For example, in a lighting system, dimmers provide an input signal to a lighting system, and the load includes one or more light sources such as one or more light emitting diodes (LEDs). Dimmers can also be used to modify power delivered to other types of loads, such as one or more motors or one or more portable power sources. The input signal represents a dimming level that causes the lighting system to adjust power delivered to a lamp, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp. Many different types of dimmers exist. In general, dimmers use a digital or analog coded dimming signal that indicates a desired dimming level. For example, phase-cut dimmers modulate a phase angle of each cycle of an alternating current supply voltage. Modulating the phase angle of the supply voltage is also commonly referred to as “chopping” or “phase cutting” the supply voltage. Phase cutting the supply voltage causes the voltage supplied to a lighting system to rapidly turn on and off thereby controlling the average power delivered to the lighting system. There are two main types of phase-cut dimmers. One is the leading-edge phase-cut dimmer, which is usually based on a triode for alternating current device (“triac”). The other is the trailing-edge phase-cut dimmer, which is typically transistor based. Leading-edge phase cut dimmers reduce power to the load by delaying the start of each half cycle. Trailing-edge phase-cut dimmers stop the cycle early, leading to a sharp trailing edge in terms of output voltage.
FIG. 1 depicts a lighting system 100 that includes an illustrative triac-based leading-edge phase-cut dimmer 102. It is important to note that FIG. 1 represents only one possible implementation of a triac-based dimmer and that numerous other implementations are possible. FIG. 2 depicts voltage graphs 200 associated with the lighting system 100. Referring to FIGS. 1 and 2, the lighting system 100 receives an AC supply voltage VSUPPLY from voltage supply 104. The supply voltage VSUPPLY, indicated by voltage waveform 202, is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. Triac 106 acts as voltage-driven switch, and a gate terminal 108 of triac 106 controls current flow between the first terminal 110 and the second terminal 112. A gate voltage VG on the gate terminal 108 will cause the triac 106 to turn ON and current IDIM when the gate voltage VG reaches a firing threshold voltage value VF and a voltage potential exists across the first and second terminals 110 and 112. The dimmer output voltage VDIM is zero volts from the beginning of each of half cycles 202 and 204 at respective times t0 and t2 until the gate voltage VG reaches the firing threshold voltage value VF. Dimmer output voltage VDIM represents the output voltage of dimmer 102. During timer period TOFF, the dimmer 102 chops the supply voltage VSUPPLY so that the dimmer output voltage VDIM remains at zero volts during time period TOFF. At time t1, the gate voltage VG reaches the firing threshold value VF, and triac 106 begins conducting. Once triac 106 turns on, the dimmer voltage VDIM tracks the supply voltage VSUPPLY during time period TON. Once triac 106 turns on, triac 106 continues to conduct current IDIM regardless of the value of the gate voltage VG as long as the current IDIM remains above a holding current value IHC. The holding current value IHC is a function of the physical characteristics of the triac 106. Once the current IDIM drops below the holding current value IHC, i.e., IDIM<IHC, triac 106 turns off, i.e., stops conducting, until the gate voltage VG again reaches the firing threshold value VF. The holding current value IHC is generally low enough so that, ideally, the current IDIM drops below the holding current value IHC when the supply voltage VSUPPLY is approximately zero volts near the end of the half cycle 202 at time t2.
The variable resistor 114 in series with the capacitor 116 form a timing circuit to control the time t1 at which the gate voltage VG reaches the firing threshold value VF. Increasing the resistance of variable resistor 114 increases the time TOFF, and decreasing the resistance of variable resistor 114 decreases the time TOFF. The resistance value of the variable resistor 114 effectively sets a dimming value for lamp 122. Diac 119 provides current flow into the gate terminal 108 of triac 106. The diode bridge rectifier 118 rectifies the dimmer voltage VDIM to generate rectified voltage VR 208. The LED load 122 comprises one or more LED lamps such as lamps 124 and 126.
Ideally, modulating the phase angle of the dimmer output voltage VDIM effectively turns the lamps 124 and 126 off during time period TOFF and on during time period TON for each half cycle of the supply voltage VSUPPLY. Thus, ideally, the dimmer 102 effectively controls the average energy supplied to the lamps 124 and 126 in accordance with the dimmer output voltage VDIM. However, when the lamps 124 and 126 draw only a small amount of current (e.g., when the lamps 124 and 126 are relatively low wattage lamps), the current IDIM can prematurely drop below the holding current value IHC before the supply voltage VSUPPLY reaches approximately zero volts. When the current IDIM prematurely drops below the holding current value IHC, the dimmer 102 prematurely shuts down, and the dimmer voltage VDIM prematurely drops to zero. When the dimmer voltage VDIM prematurely drops to zero, it does not reflect the intended dimming value as set by the resistance value of variable resistor 114. For example, when the current IDIM drops below the holding current value IHC at time t3 for the dimmer voltage VDIM 206, the ON time period TON prematurely ends at time t3 rather than at time t2, thereby decreasing the amount of energy delivered to lamps 124 and 126. Thus, the energy delivered to lamps 124 and 126 will not match the dimming level corresponding to the dimmer voltage VDIM. Additionally, the triac 106 of leading-edge dimmer 102 can re-engage (conductive) and disengage (non-conductive) repeatedly during a half-cycle of supply voltage VSUPPLY when the current IDIM is below or near the holding current value IHC.
Measures can be taken to mitigate the above-described non-ideal operating characteristics of a leading-edge phase-cut dimmer. For that reason, it would be advantageous to know when a leading-edge phase-cut dimmer is present. In general, dimmers do not directly communicate their dimmer type to other circuits that could benefit from knowing the dimmer type.